Internet Engineering Task Force (IETF) D. Wessels
Request for Comments: 8145 Verisign
Category: Standards Track W. Kumari
ISSN: 2070-1721 Google
P. Hoffman
ICANN
April 2017
Signaling Trust Anchor Knowledge in DNS Security Extensions (DNSSEC)
Abstract
The DNS Security Extensions (DNSSEC) were developed to provide origin
authentication and integrity protection for DNS data by using digital
signatures. These digital signatures can be verified by building a
chain of trust starting from a trust anchor and proceeding down to a
particular node in the DNS. This document specifies two different
ways for validating resolvers to signal to a server which keys are
referenced in their chain of trust. The data from such signaling
allow zone administrators to monitor the progress of rollovers in a
DNSSEC-signed zone.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc8145.
Wessels, et al. Standards Track [Page 1]

RFC 8145 DNSSEC Key Tag Signaling April 20171. Introduction
The DNS Security Extensions (DNSSEC) [RFC4033] [RFC4034] [RFC4035]
were developed to provide origin authentication and integrity
protection for DNS data by using digital signatures. DNSSEC uses
Key Tags to efficiently match signatures to the keys from which they
are generated. The Key Tag is a 16-bit value computed from the RDATA
portion of a DNSKEY resource record (RR) using a formula not unlike a
ones-complement checksum. RRSIG RRs contain a Key Tag field whose
value is equal to the Key Tag of the DNSKEY RR that validates the
signature.
Likewise, Delegation Signer (DS) RRs also contain a Key Tag field
whose value is equal to the Key Tag of the DNSKEY RR to which it
refers.
This document specifies how validating resolvers can tell a server,
in a DNS query, which DNSSEC key(s) they would use to validate the
server's responses. It describes two independent methods for
conveying Key Tag information between clients and servers:
1. placing an EDNS option in the OPT RR [RFC6891] that contains the
Key Tags (described in Section 4)
2. periodically sending special "Key Tag queries" to a server
authoritative for the zone (described in Section 5)
Each of these new signaling mechanisms is OPTIONAL to implement and
use. These mechanisms serve to measure the acceptance and use of new
DNSSEC trust anchors and key signing keys (KSKs). This signaling
data can be used by zone administrators as a gauge to measure the
successful deployment of new keys. This is of particular interest
for the DNS root zone in the event of key and/or algorithm rollovers
that rely on [RFC5011] to automatically update a validating DNS
resolver's trust anchor.
This document does not introduce new processes for rolling keys or
updating trust anchors. Rather, it specifies a means by which a DNS
query can signal the set of keys that a client uses for DNSSEC
validation.
Wessels, et al. Standards Track [Page 3]

RFC 8145 DNSSEC Key Tag Signaling April 20171.1. Design Evolution
Initially, when the work on this document started, it proposed
including Key Tag values in a new EDNS(0) option code. It was
modeled after [RFC6975], which provides DNSSEC algorithm signaling.
The authors received feedback from participants in the DNSOP Working
Group that it might be better to convey Key Tags in the QNAME of a
separate DNS query, rather than as an EDNS(0) option. Mostly, this
is because forwarding (e.g., from stub to recursive to authoritative)
could be problematic. Reasons include the following:
1. EDNS(0) is a hop-by-hop protocol. Unknown option codes would not
be forwarded by default, as per [RFC6891].
2. Middleboxes might block entire queries containing unknown EDNS(0)
option codes.
3. A recursive resolver might need to remember Key Tag values (i.e.,
keep state) received from its stub clients and then forward them
at a later opportunity.
One advantage of the EDNS(0) option code is that it is possible to
see that a stub client has a different Key Tag list than its
forwarder. In the QNAME-based approach, this is not possible because
queries originated by a stub and a forwarder are indistinguishable.
The authors feel that this advantage is not sufficient to justify the
EDNS(0) approach.
One downside to the QNAME approach is that it uses a separate query,
whereas with EDNS(0) the Key Tag values are "piggybacked" onto an
existing DNSKEY query. For this reason, this document recommends
only sending QNAME-based Key Tag queries for trust anchors, although
EDNS-based Key Tags can be sent with any DNSKEY query.
Another downside to the QNAME-based approach is that since the
trust anchor zone might not contain labels matching the QNAME, these
queries could be subject to aggressive negative caching features now
in development by the Working Group. This could affect the amount of
signaling sent by some clients compared to others.
A probably minor downside to the QNAME-based approach is that it
cannot be used with extremely long query names if the addition of the
prefix would cause the name to be longer than 255 octets.
Wessels, et al. Standards Track [Page 4]

RFC 8145 DNSSEC Key Tag Signaling April 20172. Requirements Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Terminology
Trust Anchor: A configured DNSKEY RR or DS RR hash of a DNSKEY RR.
A validating security-aware resolver uses this public key or hash
as a starting point for building the authentication chain to a
signed DNS response. In general, a validating resolver will have
to obtain the initial values of its trust anchors via some secure
or trusted means outside the DNS protocol. Presence of a
trust anchor also implies that the resolver should expect the zone
to which the trust anchor points to be signed. (This paragraph is
quoted from Section 2 of [RFC4033].)
Key Tag: A 16-bit integer that identifies and enables efficient
selection of DNSSEC public keys. A Key Tag value can be computed
over the RDATA of a DNSKEY RR. The Key Tag field in the RRSIG and
DS records can be used to help select the corresponding DNSKEY RR
efficiently when more than one candidate DNSKEY RR is available.
For most algorithms, the Key Tag is a simple 16-bit modular sum of
the DNSKEY RDATA. See [RFC4034], Appendix B.
4. Using the edns-key-tag Option4.1. Option Format
The edns-key-tag option is encoded as follows:
0 8 16
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| OPTION-CODE |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| OPTION-LENGTH |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| KEY-TAG |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ... /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
Wessels, et al. Standards Track [Page 5]

RFC 8145 DNSSEC Key Tag Signaling April 2017
where:
OPTION-CODE: The EDNS0 option code assigned to edns-key-tag (14).
OPTION-LENGTH: The value 2 x number of key-tag values present.
KEY-TAG: One or more 16-bit Key Tag values ([RFC4034], Appendix B).
4.2. Use by Queriers
A validating resolver sets the edns-key-tag option in the OPT RR when
sending a DNSKEY query. The validating resolver SHOULD also set the
DNSSEC OK bit (also known as the DO bit) [RFC4035] to indicate that
it wishes to receive DNSSEC RRs in the response.
A DNS client MUST NOT include the edns-key-tag option for non-DNSKEY
queries.
The KEY-TAG value(s) included in the edns-key-tag option represents
the Key Tag of the trust anchor or DNSKEY RR that will be used to
validate the expected response. When the client sends a DNSKEY
query, the edns-key-tag option represents the Key Tag(s) of the
KSK(s) of the zone for which the server is authoritative. A
validating resolver learns the Key Tag(s) of the KSK(s) from the
zone's DS record(s) (found in the parent) or from a trust anchor.
A DNS client SHOULD include the edns-key-tag option when issuing a
DNSKEY query for a zone corresponding to a trust anchor.
A DNS client MAY include the edns-key-tag option when issuing a
DNSKEY query for a non-trust anchor zone (i.e., Key Tags learned via
DS records). Since some DNSSEC validators implement bottom-up
validation, a non-trust anchor Key Tags zone might not be known at
the time of the query. Such a validator can include the edns-key-tag
option based on previously cached data.
A DNS client MUST NOT include Key Tag(s) for keys that are not
learned via either a trust anchor or DS records.
Since the edns-key-tag option is only set in the query, if a client
sees these options in the response, no action needs to be taken and
the client MUST ignore the option values.
Wessels, et al. Standards Track [Page 6]

RFC 8145 DNSSEC Key Tag Signaling April 20174.2.1. Stub Resolvers
Typically, stub resolvers rely on an upstream recursive resolver (or
cache) to provide a response. Optimal setting of the edns-key-tag
option depends on whether the stub resolver elects to perform its own
validation.
4.2.1.1. Validating Stub Resolvers
A validating stub resolver sets the DNSSEC OK bit [RFC4035] to
indicate that it wishes to receive additional DNSSEC RRs (i.e., RRSIG
RRs) in the response. Such validating resolvers SHOULD include the
edns-key-tag option in the OPT RR when sending a DNSKEY query.
4.2.1.2. Non-validating Stub Resolvers
The edns-key-tag option MUST NOT be included by non-validating stub
resolvers.
4.2.2. Recursive Resolvers4.2.2.1. Validating Recursive Resolvers
A validating recursive resolver is, by definition, configured with at
least one trust anchor. Thus, a recursive resolver SHOULD include
the edns-key-tag option in its DNSKEY queries as described above.
In addition, the clients of a validating recursive resolver might be
configured to do their own validation, with their own
trust anchor(s). When a validating recursive resolver receives a
query that includes the edns-key-tag option with a Key Tag list that
differs from its own, it SHOULD forward both the client's Key Tag
list and its own list. When doing so, the recursive resolver SHOULD
transmit the two Key Tag lists using separate instances of the
edns-key-tag option code in the OPT RR. For example, if the
recursive resolver's Key Tag list is (19036, 12345) and the
stub/client's list is (19036, 34567), the recursive resolver
would include the edns-key-tag option twice: once with values
(19036, 12345) and once with values (19036, 34567).
A validating recursive resolver MAY combine stub/client Key Tag
values from multiple incoming queries into a single outgoing query.
It is RECOMMENDED that implementations place reasonable limits on the
number of Key Tags to include in the outgoing edns-key-tag option.
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RFC 8145 DNSSEC Key Tag Signaling April 2017
If the client included the DNSSEC OK and Checking Disabled (CD) bits
but did not include the edns-key-tag option in the query, the
validating recursive resolver MAY include the option with its own
Key Tag values in full.
Validating recursive resolvers MUST NOT set the edns-key-tag option
in the final response to the stub client.
4.2.2.2. Non-validating Recursive Resolvers
Recursive resolvers that do not validate responses SHOULD copy the
edns-key-tag option seen in received queries, as they represent the
wishes of the validating downstream resolver that issued the original
query.
4.3. Use by Responders
An authoritative name server receiving queries with the edns-key-tag
option MAY log or otherwise collect the Key Tag values to provide
information to the zone operator.
A responder MUST NOT include the edns-key-tag option in any DNS
response.
5. Using the Key Tag Query5.1. Query Format
A Key Tag query consists of a standard DNS query of type NULL and of
class IN [RFC1035].
The first component of the query name is the string "_ta-" followed
by a sorted, hyphen-separated list of hexadecimal-encoded Key Tag
values. The zone name corresponding to the trust anchor is appended
to this first component.
For example, a validating DNS resolver that has a single root zone
trust anchor with Key Tag 17476 (decimal) would originate a query of
the form QTYPE=NULL, QCLASS=IN, QNAME=_ta-4444.
Hexadecimal values MUST be zero-padded to four hexadecimal digits.
For example, if the Key Tag is 999 (decimal), it is represented in
hexadecimal as 03e7.
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RFC 8145 DNSSEC Key Tag Signaling April 2017
When representing multiple Key Tag values, they MUST be sorted in
order from smallest to largest. For example, a validating DNS
resolver that has three trust anchors for the example.com zone with
Key Tags 1589, 43547, 31406 (decimal) would originate a query of the
form QTYPE=NULL, QCLASS=IN, QNAME=_ta-0635-7aae-aa1b.example.com.
5.2. Use by Queriers
A validating DNS resolver (stub or recursive) SHOULD originate a
Key Tag query whenever it also originates a DNSKEY query for a
trust anchor zone. In other words, the need to issue a DNSKEY query
is also the trigger to issue a Key Tag query.
The value(s) included in the Key Tag query represents the Key Tag(s)
of the trust anchor that will be used to validate the expected DNSKEY
response.
A validating DNS resolver SHOULD NOT originate Key Tag queries when
also originating DNSKEY queries for non-trust anchor zones.
A non-validating DNS resolver MUST NOT originate Key Tag queries.
DNS resolvers with caches SHOULD cache and reuse the response to a
Key Tag query just as it would any other response.
5.3. Use by Responders
An authoritative name server receiving Key Tag queries MAY log or
otherwise collect the Key Tag values to provide information to the
zone operator.
An authoritative name server MUST generate an appropriate response to
the Key Tag query. A server does not need to have built-in logic
that determines the response to Key Tag queries: the response code is
determined by whether the data is in the zone file or covered by
wildcards. The zone operator might want to add specific Key Tag
records to its zone, perhaps with specific TTLs, to affect the
frequency of Key Tag queries from clients.
5.3.1. Interaction with Aggressive Negative Caching
Aggressive NSEC/NSEC3 negative caching [NSEC-NSEC3-Usage] may also
affect the quality of Key Tag signaling. When the response code for
a Key Tag query is NXDOMAIN, DNS resolvers that implement aggressive
negative caching will send fewer Key Tag queries than resolvers that
do not implement it.
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RFC 8145 DNSSEC Key Tag Signaling April 2017
For this reason, zone operators might choose to create records
corresponding to expected Key Tag queries. During a rollover from
Key Tag 1111 (hex) to Key Tag 2222 (hex), the zone could include the
following records:
_ta-1111 IN NULL \# 0
_ta-2222 IN NULL \# 0
_ta-1111-2222 IN NULL \# 0
Recall that when multiple Key Tags are present, the originating
client MUST sort them from smallest to largest in the query name.
6. IANA Considerations
IANA has assigned an EDNS0 option code for the edns-key-tag option in
the "DNS EDNS0 Option Codes (OPT)" registry as follows:
+-------+--------------+----------+-----------+
| Value | Name | Status | Reference |
+-------+--------------+----------+-----------+
| 14 | edns-key-tag | Optional | RFC 8145 |
+-------+--------------+----------+-----------+
7. Security Considerations
This document specifies two ways for a client to signal its
trust anchor knowledge to a cache or server. The goal of these
optional mechanisms is to signal new trust anchor uptake in clients
to allow zone administrators to know when it is possible to complete
a key rollover in a DNSSEC-signed zone.
There is a possibility that an eavesdropper or server could infer the
validator in use by a client by the Key Tag list seen. This may
allow an attacker to find validators using old, possibly broken,
keys. It could also be used to identify the validator or to narrow
down the possible validator implementations in use by a client; the
validator used by the client could have a known vulnerability that
could be exploited by the attacker.
Consumers of data collected from the mechanisms described in this
document are advised that provided Key Tag values might be "made up"
by some DNS clients with malicious, or at least mischievous,
intentions. For example, an attacker with sufficient resources might
try to generate large numbers of queries including only old Key Tag
values, with the intention of delaying the completion of a key
rollover.
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RFC 8145 DNSSEC Key Tag Signaling April 2017
DNSSEC does not require keys in a zone to have unique Key Tags.
During a rollover, there is a small possibility that an old key and a
new key will have identical Key Tag values. Zone operators relying
on the edns-key-tag mechanism SHOULD take care to ensure that new
keys have unique Key Tag values.
8. Privacy Considerations
This proposal provides additional, optional "signaling" to DNS
queries in the form of Key Tag values. While Key Tag values
themselves are not considered private information, it may be possible
for an eavesdropper to use Key Tag values as a fingerprinting
technique to identify particular validating DNS clients. This may be
especially true if the validator is configured with trust anchors for
zones in addition to the root zone.
A validating resolver need not transmit the Key Tags in every
applicable query. Due to privacy concerns, such a resolver MAY
choose to transmit the Key Tags for a subset of queries (e.g., every
25th time) or by random chance with a certain probability (e.g., 5%).
Implementations of this specification MAY be administratively
configured to only transmit the Key Tags for certain zones. For
example, the software's configuration file may specify a list of
zones for which the use of the mechanisms described here is allowed
or denied. Since the primary motivation for this specification is to
provide operational measurement data for root zone key rollovers, it
is RECOMMENDED that implementations at least include the edns-key-tag
option for root zone DNSKEY queries.
9. References9.1. Normative References
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <http://www.rfc-editor.org/info/rfc1035>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, DOI 10.17487/RFC4033, March 2005,
<http://www.rfc-editor.org/info/rfc4033>.
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